This invention relates to solid lubricant systems which exhibit stable lubricity over a wide temperature range.
Solid lubricants are required in applications where components are exposed to high temperatures, vacuum, intense radiation and other degrading environments. In particular, use of those components in precision applications (e.g., close tolerance gimbals, bearings, shafts and gears) requires a uniform thin coating. The space industry utilizes a significant number of precision components whose lubricant supply is not easily replenished after launch. Lubricant failure in systems requiring those components could cause mission failure. Increased demands on lubricant life are also found in high temperature engines and systems whose components are sealed and not readily accessible. There is a great need, therefore, to extend the wear life and environmental stability of solid lubricants used in the applications discussed above. Currently, solid lubricants are available that perform well in the low (-196.degree. to +350.degree. C.), intermediate (350.degree. C. to 700.degree. C.) and high (above 700.degree. C.) temperature ranges. However, no lubricant is available that effectively spans all three ranges.
Molybdenum disulfide (MoS.sub.2) has been a widely used solid lubricant for many years because it is readily available, provides a low friction coefficient, can bear high loads, and is somewhat resistant to oxidation up to about 350.degree. C. Other dichalcogens have been used at temperatures up to 350.degree. C. with varying degrees of success. PbO is useful in the intermediate temperature range from 350.degree. C. to 700.degree. C. and CaF-BaF mixtures provide lubrication at temperatures above 700.degree. C.
Many methods exist for depositing lubricants; for example, they can be: rubbed, bonded (using an adherent polymer matrix), thermally sprayed, laser deposited, and sputtered onto components requiring lubrication and/or wear protection. Sputter deposition is currently the most widely used method to coat precision components with solid lubricants and the chemical, physical and mechanical properties of those coatings have been well characterized.
While sputter deposition is used to grow technologically important lubricant films, there are aspects of the sputter process that limit film quality. The most significant deficiencies are: contamination by background gases (i.e., H.sub.2 O, O.sub.2, etc.), contamination by elements used in prior depositions, and porous morphology. Contamination from any source can compromise film integrity and prohibit good control of film properties. The porosity and morphology of some sputter deposited films exposes reactive edge planes to the environment where they are susceptible to degradation by oxidants.
Pulsed laser deposition (PLD) of lubricants provides coatings that are free from most deficiencies discussed above. The relative merits of PLD films are well known. One avenue used to increase the wear life of lubricant coatings is to incorporate additives. Graphite, Sb.sub.2 O.sub.3, Ni and other metals have been combined with MoS.sub.2. The limitation of current lubricant and deposition technology is that a lubricant that effectively spans a broad temperature range cannot be produced.
Accordingly, it is an object of this invention to provide an improved lubricant coating capable of performing over a temperature range of 0.degree. to 600.degree. C., preferably -196.degree. to 700.degree. C. or greater.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art after reading the detailed description of the invention as well as the appended claims.